The Zippe-type centrifuge is a device designed to collect Uranium-235. It was developed in the Soviet Union by a team of 60 German scientists working in detention, captured after World War II. The centrifuge is named after the team's lead experimenter, Gernot Zippe.
Natural uranium consists of two isotopes; the majority (99.3 percent) is U-238, while approximately 0.7 percent is U-235. If natural uranium is enriched to contain 5 percent U-235 it can be used as fuel for light water nuclear reactors. At 90 percent it can be used for nuclear weapons.
Enriching uranium is difficult because the two isotopes are very similar in weight: U-235 is only slightly lighter. It requires a centrifuge that can spin at 1,500 revolutions per second (90,000 RPM). For comparison, washing machines operate at only 15 revolutions per second.
The device has a hollow, cylindrical rotor filled with gaseous uranium. A pulsating magnetic field at the bottom of the rotor, similar to that used in an electric motor, is able to spin it quickly enough that the U-238 is thrown towards the edge. The lighter U-235 collects in the centre. The bottom of the gaseous mix is heated, producing currents that move the U-238 down. The U-235 moves up, where scoops collect it.
To reduce friction the rotor spins in a vacuum. A magnetic bearing holds the top of the rotor steady, and the only physical contact is the needle-like bearing that the rotor sits on.
After the captured scientists were released, Gernot Zippe was surprised to find that engineers in the West were years behind in their centrifuge technology. He was able to reproduce his design in the United States, even though the Soviets had confiscated his notes. In the 1960s, Dr. Zippe and his colleagues made the centrifuge more efficient by changing the material of the rotor from aluminium to an alloy called maraging steel, which allowed it to spin even faster without risk of it tearing itself apart.
The Zippe-type is difficult to build successfully, and it requires very carefully machined parts. However, compared to other enrichment methods, it is much cheaper and can be used in relative secrecy. This makes it ideal for covert nuclear weapons programs and possibly increases the risk of nuclear proliferation.
In 2004 Abdul Qadeer Khan, a Pakistani engineer, admitted to operating a smuggling ring responsible for supplying at least three countries with Zippe-type centrifuges.
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